1. Field of the Invention
The present invention relates to a contour correcting circuit for sharpening rising edges and falling edges of video signals for improvement of image sharpness on picture.
2. Description of the Prior Art
Conventionally, there has been used a contour correcting circuit for sharpening the rising and falling edges of video signals in order to improve the obscurity and the lack of resolution of video signals of a TV set, for instance.
FIG. 1 is a block diagram showing an example of the conventional contour correcting circuit. The construction and the operation of this contour correcting circuit will be described hereinbelow with reference to FIG. 2A. FIG. 2A shows the respective waveforms of signals a to g at the respective portions of the circuit shown in FIG. 1.
In FIG. 1, a signal a inputted to an input terminal 1 is applied to a delay circuit 2. The delay circuit 2 outputs a delay signal b delayed by a predetermined time from the signal a. The delayed signal b is applied to another delay circuit 3. The delay circuit 3 outputs another delay signal c delayed by another predetermined time from the signal b.
Further, the signals a and b are applied to a computing circuit 4. The computing circuit 4 subtracts the signal a from the signal b, and outputs a signal d (=b-a). In the same way, the signals b and c are applied to another computing circuit 5. The computing circuit 5 subtracts the signal c from the signal b, and outputs a signal e (=b-c).
These two signals d and e are inputted to an adder 6. The adder 6 adds the two signals d and e, and outputs a signal f (=d+e) (a contour correcting signal). Further, the signal f and the signal b are inputted to another adder 7. The adder 7 adds the two signals f and b, and outputs a signal g (=f+b) (a contour corrected signal).
FIG. 2A indicates that the rising and falling edges of the output signal g are sharpened in comparison with those of the input signal a, so that it is possible to obtain a video signal whose contour is corrected or emphasized so as to provide a sharp image on picture.
In the above-mentioned conventional contour correcting circuit, however, as shown by the waveform g shown in FIG. 2A, preshoots and overshoots (or undershoots) are generated at the edge portions of the output signal g due to the sharpness correction. However, when the sharpness is further required to be increased by sharpening both the rising and falling edges of the output signal g, the addition rate of the contour correcting signal f to the signal b must be increased. In the case where the amplitudes of the rising and falling edges of the contour correcting signal f (as shown in FIG. 2A) are doubled as shown by the signal h in FIG. 2B, since the signal b shown in FIG. 2A is added to the amplitude-doubled contour correcting signal h as shown in FIG. 2B (instead of the signal f shown in FIG. 2A), the output signal i as shown in FIG. 2B is obtained. The obtained signal i is high in both the preshoot and overshoot levels, so that the edges of the output signal i are more sharpened than those of the signal g.
However, when the preshoot and the overshoot levels are emphasized excessively, the contour portions looks as if being bordered in white or black on picture, so that the picture is not natural. Therefore, it is preferable to reduce the preshoot and overshoot levels as much as possible. In the conventional contour correcting circuit, however, when the sharpness is improved by increasing the rising and falling rates of the image signal, since the preshoot and overshoot levels increase with increasing addition rate of the contour correcting signal g to the signal b, an artificiality or an unnaturalness becomes prominent in the corrected image. In other words, in the conventional contour correcting circuit, there exists a problem in that it is impossible to improve the sharpness without increasing the harmful influence of the preshoot and overshoot levels upon the picture quality.